The power output of high-speed, low torque dental handpieces, such as air turbine handpieces, can be measured using a string tension dynamometer. For example, it is known to use a Kerfoot string tension dynamometer, which is a device that applies a load to a handpiece through a string looped around a pulley mounted in a handpiece chuck (See FIG. 1). With a Kerfoot device, string tension is measured by the deflection of weighted dials to which the ends of the string are attached. Under steady-state conditions, the net tension on the string multiplied by the pulley radius is equal to the handpiece torque.
According to one test protocol, the face of the pulley is half blacked out for speed detection using an optical tachometer. The pulley shaft conforms to DIN 13950 and ISO 1797 (0.0628″, 1.595 millimeters diameter). The pulley wheel is lightweight aluminum, unconcentricity not more the 0.0003 inches. Each pulley is tested for balance at speeds up to 500,000 RPM by recording the free spin RPM of a new handpiece with each pulley and discarded if they are statistical outliers. The maximum power output of a handpiece occurs at a speed that is about half the no-load (or free-running) speed. To determine the power output, the maximum speed and the torque at half the maximum speed is measured. Torque and power can be measured as follows:P=vT, where v=RPM (2π)/60 and T=(TR-TL)mgR                P is the power in Watts.        v is the speed expressed as angular velocity, radians per second.        RPM is the speed in revolutions per minute at which the torque was measured.        π is the constant 3.14159.        T is the torque expressed as Newton-meters.        TR and TL are the right and left dial deflection readings (See FIG. 1).        m is the mass of the dial weights expressed in kilograms.        g is the gravitational acceleration, about 9.8 meters per second2.        R is the pulley radius in meters (such as for example, 0.100″ or 2.54×10−3 meters). The actual pulley radius is adjusted to compensate for the thickness of the string. The effective pulley radius including the radius of the string is 0.100″.        
While such dynamometers have proven valuable in determining power, they do result in a small lateral force being exerted on the test wheel. Therefore, a purely torsional load never exists resulting in increased measurement error. A need exists therefore, for a string tension dynamometer which will avoid the torsional load-induced errors.